Field of the Invention
The invention relates to a unipolar semiconductor laser component, including a semiconductor body with a hetero-structure configuration, in particular an SCH (Separate Confinement Heterostructure) configuration, which is suitable for generating an electromagnetic radiation. In the configuration, an active layer sequence with a quantum well structure is provided between a first outer cover layer of a first conductivity type and a second outer cover layer of the first conductivity type above a semiconductor substrate. The electromagnetic radiation is generated within the quantum well structure (or active zone) when current flows through the semiconductor body. The cover layers have a lower index of refraction than the active layer sequence, and as a result an optical wave being generated during operation is confined between the cover layers. A laser diode is described in a paper entitled "Carrier and Photon Dynamics in Transversally Asymmetric High-Speed AlGaInAs/InP MQW Lasers", by H. Hillmer, A. Greiner, F. Steinhagen, H. Burkhard, R. Loesch, W. Schlapp and T. Kuhn, in Physics and Simulation of Optoelectronic Devices IV, SPIE, Vol. 2693 (1996), pp. 352-368. In that laser diode, an n-conducting cover layer of InP and over that an active layer sequence formed of InGaAs/AlInGaAs and a further cover layer of p-conducting InAlAs, are applied over a semiconductor substrate of semi-isolating or n-conducting InP. The active zone is formed of InGaAs multiple quantum wells (MQWs) which are embedded in AlInGaAs barriers and AlInGaAs waveguide layers. The AlInGaAs waveguide layers are both p-conducting, but at p=5*10.sup.17 cm.sup.-3, have lesser doping than the cover layers of InP (where n=2*10.sup.18 cm.sup.-3) or InAlAs (where p=2*10.sup.18 cm.sup.-3). The configuration disclosed by Hillmer et al is a so-called separate confinement heterostructure (SCH), in which electrons in holes are injected into an active zone through a pn junction which is formed substantially of the cover layers, in that case n-InP and p-InAlAs. The optical wave being generated is carried, regardless of the charge carrier confinement in the quantum wells, through the waveguide, which is surrounded by the cover layers with the lower index of refraction. In that special case, the active MQW zone is not symmetrical, or in other words placed centrally in the waveguide. The waveguide is shortened on the side toward the p-conducting cover layer, in order to speed up the transport of holes into the quantum well, since holes in principle have markedly lesser mobility than electrons. Improved modulability is thus attained, which is determined essentially by the transport of less mobile holes within the moderately doped waveguide and by the electron confinement in the MQW structure.
The basic structures of quantum well and MQW semiconductor lasers and of the SCH configuration are described, for instance, in a book entitled "Die physikalischen Grundlagen der LED's, Diodenlaser und pn-Photodioden" [The Physical Basis of LED's, Diode Lasers and pn-Photodiodes] by W. Buldau, in Halbleiter-Optoelektronik [Semiconductor Optoelectronics], Hanser-Verlag, Munich and Vienna, 1995, pp. 182-187, and are therefore not described in further detail herein.
In lasers based on the materials InGaAsP and AlInGaAs that have been tested thus far, the hole mobility in the waveguide layer is markedly less than the mobility of electrons.
In order to provide a high data transmission rate at wavelengths of 1.3 .mu.m and 1.55 .mu.m in the optical window of conventional glass fibers, a limitation to high-frequency modulability is presented by the drift transport of less-mobile holes into the quantum well and the electron confinement in the MQW structure.
Due to the aforementioned shortening of the waveguide layer on the p side, the transport length for holes in the waveguide can be shortened. However, the drift speed is determined by the electrical field applied and by the poor mobility.